It is known in the art to employ rack gears and pinions for effecting longitudinal movements and further to provide a reciprocal carrier which moves between a pair of opposed rack gears with one rack gear being stationary and the other being mounted upon a movable carriage, for effecting longitudinal movements of said carriage. One example of such device is shown in U.S. Pat. No. 3,665,771 of May 30, 1972.